Water jet propulsion systems are becoming increasingly popular. Many skiers, fishers and family boaters realize that water jet propelled craft offer safety, simplicity and diverse utility. The propulsive force of the water jet transfers directly into the water without going through gears, right-angle shafts, or clutches. This translates into less weight, lower cost, lower maintenance and more reliability when compared to standard marine propeller drives.
Functionally, a water jet propulsion system, or marine jet drive, is simply a propeller inside a pipe. The propeller operates as a pump impeller or rotor. The propulsion water intake port is typically an opening near the bottom of the hull, which picks up water and delivers it to the jet pump impeller. A grill across the intake port prevents foreign matter from entering the system. Water jet propulsion systems are also available in outboard engine packages for water craft. Simply stated, all marine jet drives function by inhaling water, compressing it, and passing it out a nozzle in the stern. The result is a powerful jet of water that pushes the vessel forward in the water.
A water jet propulsion system can operate in just inches of water, as there is no external propeller. Jet driven water craft can skim flats, thread treacherous rocky passages and navigate river shoals. However, running these craft in extremely shallow water can disturb the bottom environment, clogging the jet with sand, gravel, weeds, litter or any other material bypassing the propulsion water intake grill.
A variety of problems result when the propulsion water intake becomes clogged. When the obstruction is only partial, the operator may notice a slight loss of power. If the cause of the reduced power is misinterpreted, the operator may compensate by increasing the fuel feed to the jet drive engine. The engine responds to the increase in fuel by increasing its revolutions per minute (R.P.M.). At a minimum, partial blockage of the propulsion water intake results in poor fuel economy and unnecessary wear on the engine from the over-revving. Serious engine damage occurs from prolonged or severe over-revving.
A fully blocked propulsion water intake generally results in cavitation, or boiling of the propulsion water. Cavitation is produced by near-vacuum suction around the impeller. The resultant vapor bubbles in the water reduce the load on the engine, so that it over-revs. Additionally, because of the increase in suction caused by the blockage, the material clogging the propulsion water intake can be drawn into the jet impeller, resulting in possible permanent damage to the impeller. Such damage may require underwater repairs, which are difficult and costly. Even brief failure of propulsion water flow can cause over-revving, which ultimately results in engine damage or failure.
In rough or "choppy" water there is an immediate "blowout," or loss of power, when the jet drive comes out of the water. The operator of a jet drive water craft must be able to quickly diagnose the cause of the power loss, as swift corrective measures may be essential to safe docking or maneuvering.
The occasional failure of propulsion water flow is nearly impossible to avoid. This is especially true because most marine jet propulsion systems rely on sea or take water, drawn into the system via the propulsion water intake. Although this design provides an unlimited supply of propulsion water, there is a significant chance that waterborne debris, seaweed, dirt or dissolved minerals will cause problems in these water jet propulsion systems. Such water borne materials can clog or foul the propulsion water intake, the propulsion pipe or the outlet nozzle. Damage to the impeller is also likely, leading to propulsion system failure.
It is known to monitor marine engine over-revving using R.P.M. indicators. However, such indicators do not discern whether the over-revving is due to propulsion system failure, or is attributable to another cause, such as decreased engine load. Consequently, troubleshooting is more complicated and time consuming, and can result in unnecessary alarm over innocuous over-revving events, or inattention to serious over-revving problems.
Also, reliance on the engine R.P.M. indicator is unwise because engine damage can occur so rapidly after propulsion failure that engine sensors may not register the problem until it is too late to avoid engine damage.
The poor reliability of engine R.P.M. indicators for monitoring marine propulsion system function is widely recognized. To overcome this problem, most marine engine operators currently monitor their propulsion system's function directly, by visually inspecting the output of propulsion water from the jet drive. In a boat, this typically requires that the operator leave the helm, walk to the stern of the vessel, and peer over the rail to view the water stream. On a jet ski, the operator must turn around, while the jet ski is in motion, to observe the water stream. These actions result in obvious personal and traffic safety hazards. Furthermore, the propulsion water stream is often not observable due to rough waters, darkness, or physical obstructions such as a stern mounted swim step. Also, the inspection may be omitted due to operator inadvertence or activity conflicts. Finally, visual inspections of propulsion system function are by nature highly subjective and prone to inaccuracy. Low to intermediate propulsion water flow levels may be interpreted by an inexperienced water craft operator as adequate, although such levels may reflect critical impairment of the propulsion system.
Preventive maintenance is currently the only reliable means to insure the impeller or pump remains functionally intact. Impellers, rotors and propellers are periodically replaced at considerable expense for fear that they may soon fail.
A need exists for the monitoring of the long term performance of the water jet propulsion system by measuring the water outflow flow rate. Small changes in flow rate suggest the need for inspection or replacement. If a reliable assessment of the water jet propulsion system integrity was available, then the water craft operator could accurately determine the functional efficiency of the water jet propulsion system during normal operations. With this information, a water craft operator could safely approach, or even exceed, the advertised service life of the impeller, as long as it still functioned at an acceptable level of efficiency.
A further need exists for the monitoring of the propulsion water flow rate from a water jet propulsion system. The decrease over time of the rate of water propelled through the jet at a constant engine load can suggest a developing problem with the jet propulsion system. The verification of uninterrupted flow of propulsion water through a water jet propulsion system is essential to its optimal operation.
A related need exists for a method and apparatus for monitoring a water jet propulsion system function that distinguishes propulsion system impairment from other potential causes of marine engine over-revving.
A need also exists for a method and apparatus for monitoring water jet propulsion system function that employs direct monitoring of propulsion water flow through the water jet propulsion system.
A further need exists for the monitoring of the long term performance of the impeller or rotor for the water jet propulsion system. Small changes in flow rate suggest the need for inspection or replacement, and under normal operation, the service life of the impeller could be safely approached or exceeded if a reliable assessment of impeller integrity was available.